APA = American Psychological Association. They recently published the 6th edition of their their manuscript publication guidelines which includes a lot of typographic recommendations, most of which are downright hilarious. There have been a lot of changes for the better in this edition, but going back to a double spaced period is a real head-scratcher. Even the average office admin knows better. You could do an entire thesis on validating the efficiency of these guidelines, but I believe someone has already published a paper on that.

apa.cls is not in any way connected to, endorsed by, or probably known at all at the American Psychological Association. APA does not even accept manuscript submissions in LaTeX source. The author of apa.cls has made every effort to achieve typesetting results conforming to the APA manual, because he uses it for his manuscripts. However, he is not responsible if the editor does not approve of a manuscript typeset with apa.cls, if your computer blows up while processing your manuscript with apa.cls, or for anything else that may go wrong with apa.cls, or with anything else for that matter.

The practice of double spacing after periods is in the US known variously as "English" or "French" spacing, and has been common at one time in those countries, as well as the US. It is not an American habit today, as most style guides, I believe, are against it. The one Christopher refers to is an outlier. I also can't remember ever seeing printed material like that. It used to be standard practice in typewritten material, where it has some rationale.

I was just having a look at Appendix F of Knuth's The TeXbook, Computers and Typesetting and, even if TeX "ignores spaces" in the input, TeX fonts contain a parameter that fixes the extra space added to the interword space at the end of a sentence. For the Computer Modern (10pt) font, the interword space is 3.33pt, and the extra space is 1.11pt; however, for the Computer Modern Typewriter font, the extra space is equal to the interword space, 5.25pt, giving in fact two spaces after a period. Here is how it looks with LaTeX. The inputs are essentially the same, one with \textrm (Roman font), the other with \texttt (the typewriter font):

Input:
Output:
Michel

[added] The number of spaces in the input is irrelevant. Many spaces give the same output as one space.

David, according to this pdf the MLA style is "single space after all punctuation." The Chicago Manual of Style is the same.

I don't think you will find many style guides that still have the old double spacing, which anyway was specified I think mainly for typed manuscripts, not print. Charles can tell us more authoritatively.

The break between one sentence and the next is already clearly indicated by not one but two conventions: a fullstop at the end of one sentence, and capitalisation of the beginning of the next. That's already one more indicator than most other textual divisions. There's absolutely no need to add a third indicator by doubling the word space.

I agree with no extra space, but I think it is more a matter of custom, and less of principle, than I thought the first time around on this topic. For example, John, you could argue that within a sentence an abbreviation with a period can be followed by a cap, so to distinguish sentences definitively extra space helps. To me the holes it puts in the text are unattractive, so I go for keeping the current rule of no extra space between sentences. But as I say, I suspect it is more what I am used to reading, rather than a matter of principle.

@ bojev: Correct. For example, in APA v5, there was no such thing as bold to establish hierarchy. It is only in v6 that we are "allowed" to use bold for the first time. I still can't figure why they took so many steps forward with version, but a step backwards with the whole period thing.

I may end up working with Protopapas on updating the apa.cls from v5–6 at which point I should have a complete list of the APA's typographic conventions. A fun exercise (avoidance behaviour).

"he is not responsible if the editor does not approve of a manuscript typeset with apa.cls, if your computer blows up while processing your manuscript with apa.cls, or for anything else that may go wrong with apa.cls, or with anything else for that matter."

@Michel Boyer
That is quite possibly the best disclaimer I have ever read. It should probably be the default disclaimer on any MS product :p

JH>There’s absolutely no need to add a third indicator by doubling the word space.

All depends on whether the emphasis is on reading or editing.
For reading, you're right. It's ridiculous. But for marking it up by hand during editing, it might make it easier to mark off a particular sentence without messing up the next.
That might be the thinking, as strange as it might seem.
Giving ease of editing precedence over the readability of the finished product.
APA also calls for double spacing. Also silly in the final product. But when you're editing on paper, that extra space comes in mighty handy.
(I do quite a bit of editing of APA papers.)
That said, it's not going to be enforced by anyone or anything. In spite of what v6 might say. APA-specific software doesn't enforce it and those apps don't get upgraded too often.

I miss two spaces mainly because of the sound. Each sentence _sounds_ like it has more finality when you hit that keyboard. Multiply that by a room full of writers, and you have more diverse rhythms. (And you can tell who types faster.)

As I mentioned before in a similar thread, kerning of the difficult cap-lowercase combinations (in Linotype two-letter "logotypes" at first) rendered "double spacing" unnecessary.

Previously, if only a single space occurred before a "T_o" or similar combination, that didn't sufficiently associate the capital with its word.

Even today, double-spacing improves the appearance of certain types that have kerning between the space and letter characters. For instance, when the period at the end of a sentence is kerned under an "f", and the following sentence begins with a T.

What Americans call a "period" is known in Britain as a "full stop". In British usage a comma, being a partial stop is followed by a single space. The full stop takes two spaces both to differentiate and emphasise. This was of importance when typesetting was less precise that it is now and text was set quite tight.

Apparently there are so many mistakes in the first run of the APA 6th edition they are doing a reprint/recall/refund for anyone who bought one. Our lab will be doing so. I'll definitely double-check to see if the double-space holds up.

The practice of using a single space after the period goes back to ye Olden Dayes of dedicated phototypesetting. These machines DID NOT strip double spaces so, in order to prevent the following scenario, a single space after a period became the norm.

If a single space following a period fell at the end of a line, the space became a line break. If a double space after a period came at the end of a line, one or the other of the spaces became a line break, which meant that there was an additional, unwanted space either at the end of the first line or at the beginning of the second.

Having people pop up out of nowhere to recommend TeX is like having Objectivists pop up out of nowhere to remind us what the only role of government is.

Also: Source on your recall statement, Christopher?

Also₂: HTML rules on whitespace collapse make it impossible to end a sentence with two spaces unless one or more of them is nonbreaking. In fact, you can end sentences with a hundred spaces, tabs, and carriage returns and they’ll all be collapsed to a single space (modulo certain CMSs and blog platforms that try to turn such runs into BRs).

joeclark said:
> HTML rules on whitespace collapse
> make it impossible to end a sentence with two spaces
> unless one or more of them is nonbreaking.

i noticed something odd in an .html doc one day,
and explored to find that the person had changed
double-spaces at the end of sentences to a combo
of plainspace-nonbreakingspace, which meant that
when that beginning plainspace fell at the end of a line,
the nonbreakingspace would be the first "character" on
the next line, and would dutifully present as a space,
causing a weird void to appear at the start of the line.

joeclark said:
>> HTML rules on whitespace collapse
>> make it impossible to end a sentence with two spaces
>> unless one or more of them is nonbreaking.

>when that beginning plainspace fell at the end of a line,
>the nonbreakingspace would be the first “character” on
>the next line,

The four per-em space is pretty close in width to the space. Two of them after the period does a pretty good job of approximating two spaces. In fact, I think a four per-em space followed by a six per-em space is right on the money width-wise but not sure.
Also, using the zero-width space might prevent the wrapping to the next line of the last space character that you observed. If it doesn't help, the only answer, I think, is to get CSS in on the act.
Try this:
&#8197;&#8197;&#8203;

Nice link Craig. It's been a while since I checked in. Looked through my latest APA manual and indeed “Spacing twice after punctuation marks at the end of a sentence aids readers of draft manuscripts.” pp 88.